Employee Bargaining Agreements and Benefit Plans - Plan's Assets Carried at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	$ 11	$ 16
Annuity Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	6
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	5	5
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements		6
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements		5
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	1	12
Level 1 [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	1	1
Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements		6
Level 1 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements		5
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	4	4
Level 2 [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	4	$ 4
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	6
Level 3 [Member] | Annuity Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value measurements	$ 6